Posted by: Ken Smith on August 13, 2005 at 08:20:46:
In Reply to: Plank attachment: two studs, or one? posted by Geoff Sobering on August 10, 2005 at 17:25:40:
Geoff:
I have seen several methods of attachment and will describe some below. There is lots of room for innovation as there are no rules about this detail, as I read the rules. However, simple and reliable solutions are usually best. The attachment method should meet several design objectives: #1) It should hold the plank securely in position square to the centerline of the hull through all maneuvers and normal sailing conditions. #2) It should accommodate flexure of the plank. As the plank flexes, two points on top of the plank near the sides of the hull get closer together by about 6-8mm (1.4 in). #3) Sudden stopping of one runner is a likely failure mode (encounters with shell ice, hole, or other boat). If this were to occur, the plank-hull connection should release in some manner which is either non-destructive or at least easily repairable. #4) It should include at least two (some prefer several) possible positions for the plank within the allowable range. Forward is for hiking power and maneuverability, aft is for a light windward runner and straight-line stability.
Original design: Everything was bolted together with angle brackets and through-bolts. This met design objective 1 very well. Flexible planks (a later innovation) were a problem. It did not meet 3 very well, but could have enough mounting locations to meet 4. Now, seldom seen except in antiques and the new-to-the-sport. Not found in any current plans.
Sarns methods/Common US practice: plate fastened to hull with wood screws, stud plate on the plank, also fastened with wood screws; one or two studs per stud plate to engage the stud plate. Nuts fasten the two plates together. One side of the hull plates uses elongated stud receiver holes, so as plank flexes, stud plate allows stud to move inboard on one side. This method meets the objectives. Designed failure mode usually results in fractured screw heads or pulled-out screws, which are readily repairable. Unintended failure mode is fatigue or shear failure of the studs. (I’ve been there. One stud fatigued and let go in a snowy day, resulting in an unexpected capsize during a race.) Two studs a side are more reliable, but requires custom hardware in both plates.
Close alternatives: A) Hold the hull to the plank with nylon nuts on the stud plates. B) The studs are pins, hold the plank on with rope and jam cleat or shock cord. All weight and mast forces hold the two together anyway. C) Studs are pins, hold the plank on with heavy duty nylon tie-wraps.
Through bolt, two bolts: A sleeve is mounted in the hull on each side through the bottom of the boat at each plank location. A hole is drilled in the plank (one side elongated). A pan-head bolt is inserted through the cockpit floor and the plank holes. This meets all the objectives. On planned failure, the sleeve through the cockpit floor pulls out. Repair by gluing it back in. I had a bad experience in a boat like this when the sleeve pulled out at the leeward mark in Estonia. Simply a matter of sleeve design and glue, to remedy the slight deficiency in design. Moving the plank at and re-mounting the plank through a different hole set kept the boat in the regatta.
Plank chocked between wood brackets. The Dutch seem to prefer this. If you down load the Optimist DN plans, there is a good detail of the method. The plank is restrained fore and aft and also side to side with wood blocks glued to the hull bottom and plank outside the cockpit. Hull and plank are held together with either a center bolt or with rope and cleats, or other means. This meets all the design objectives. If designed to allow changes in plank position, then the plank and a spacer are in place between the blocking on the hull. Spacer forward means plank aft, and vice versa. Planned failure results in a split block of wood, repairable with a plane, hunk of wood and glue. If a center bolt is used, usually the planned failure results in a rotated plank. Depending on the cause and energy causing the plank to rotate, there can be additional hull damage where side runner strikes the hull or when the center bolt is pulled out.
Mike O’Brien method. Mike has a fiberglass/carbon clam shell that is bolted to the hull bottom and restrains and captures the plank. He uses nylon tie-wraps to hold the clam shell closed. Meets all objectives except #4. Some others never adjust plank location either, anyway. Exceeds all other designs in objective #3. If you hit something hard, the plank comes of. Fix it by putting it back on with a new tie-wrap. You have to talk to him for details.
Mixed and alternative Combinations. Some combine the design features of several methods. Examples: Stud plates, but plank held on with a center bolt. (may have trouble with #3). Use bicycle cleats and receivers (not unlike ski bindings). Spring adjustment required. Vertical angle clips and a stay-lock pin.